Prep the base: Add onion, carrots, celery, bell pepper, garlic, diced tomatoes, oregano, basil, thyme, red pepper flakes, bay leaf, and a pinch of salt and pepper to the crockpot. Pour in the chicken broth and stir.
Nestle in the chicken: Place chicken breasts or thighs into the mixture.
Make sure they’re mostly submerged.
Cook low and slow: Cover and cook on Low for 6–7 hours or on High for 3–4 hours, until the chicken is tender and cooked through.
Shred the chicken: Remove the chicken to a cutting board. Shred with two forks into bite-size pieces and return it to the pot. Discard the bay leaf.
Add pasta or beans: If using pasta, stir it into the crockpot and cook on High for 15–25 minutes, until al dente.
If using beans, add them now and let them warm through for 10 minutes.
Stir in the greens: Add spinach or kale and cook for 5–10 minutes, just until wilted and vibrant.
Finish with brightness: Stir in lemon juice, Parmesan, and olive oil if using. Taste and adjust with more salt and pepper as needed.
Garnish and serve: Ladle into bowls and top with extra Parmesan and chopped parsley. Serve with crusty bread if you like.